Such a technique is known that a storage reduction NOx catalyst (NOx storage reduction catalyst) (hereinafter referred to as “NSR catalyst” as well) is arranged at an exhaust gas passage of an internal combustion engine. The NSR catalyst occludes (absorbs or stores) NOx contained in the exhaust gas when the oxygen concentration of the inflow exhaust gas is high, while the NSR catalyst reduces the occluded NOx when the oxygen concentration of the inflow exhaust gas is decreased and the reducing agent is present.
The sulfur component (SOx), which is contained in the fuel, is also occluded by the NSR catalyst in the same manner as NOx. SOx, which is stored or occluded as described above, is hardly released as compared with NOx, and SOx is accumulated in the NSR catalyst. This phenomenon is referred to as “sulfur poisoning”. The NOx purification rate of the NSR catalyst is lowered or decreased by the sulfur poisoning. Therefore, it is necessary to apply the sulfur poisoning recovery process at any appropriate timing. The sulfur poisoning recovery process is performed such that the exhaust gas, which is obtained by the combustion at a rich air-fuel ratio in the internal combustion engine, is allowed to flow through the NSR catalyst.
In this context, such a technique is known that the sulfur poisoning recovery process is carried out when the SOx amount allowed to flow into the NSR catalyst exceeds a predetermined amount, and a substantially total amount of the accumulated SOx is released (see, for example, Patent Document 1).
Further, such a technique is known that the timing, at which the sulfur poisoning recovery process is carried out, is determined on the basis of the maximum NOx amount capable of being occluded by the NSR catalyst (see, for example, Patent Document 2).
Further, such a technique is known that the NOx catalyst is divided into a plurality of portions in the flow direction of the exhaust gas, the deterioration state is calculated for each of the portions, and the deterioration state of the entire NOx catalyst is calculated from the deterioration states of the respective portions (see, for example, Patent Document 3).
Further, such a technique is known that a series of control operations, in which the air-fuel ratio of the exhaust gas allowed to flow into the NOx catalyst is switched from the lean side to the rich side, the air-fuel ratio is once returned to the lean side for a predetermined time, and the air-fuel ratio is switched to the rich side again, are executed at least once, and thus the sulfur component accumulated in the NOx catalyst is purified (see, for example, Patent Document 4).
Further, such a technique is known that the purification of the sulfur component accumulated in the NOx catalyst and the removal of PM collected by a filter disposed on the downstream side from the NOx catalyst are performed simultaneously by changing the air-fuel ratio of the exhaust gas allowed to flow into the NOx catalyst from the lean side to the rich side, and the target air-fuel ratio is changed from the first target air-fuel ratio as the rich air-fuel ratio to the second target air-fuel ratio as the air-fuel ratio lower than the above if the air-fuel ratio of the exhaust gas allowed to flow into the filter is lower than a predetermined threshold value (see, for example, Patent Document 5).
In the meantime, in order to release the total amount of the sulfur component occluded by the NSR catalyst during the sulfur poisoning recovery process for the NSR catalyst, it is necessary that the air-fuel ratio of the exhaust gas should be lowered, for example, to about 12.5. Therefore, it is feared that the amounts of emission of HC and CO may be increased. Further, when the air-fuel ratio of the exhaust gas is lowered, it is feared that the fuel efficiency (fuel consumption) may be deteriorated thereby. Further, when the sulfur poisoning recovery process is carried out while lowering the air-fuel ratio of the exhaust gas, it is also feared that H2S may be produced. For the reason as described above, the sulfur poisoning recovery process is carried out for the NSR catalyst while setting the air-fuel ratio of the exhaust gas, for example, to about 14.3. That is, the sulfur poisoning recovery process is carried out at the air-fuel ratio which is approximate to the theoretical air-fuel ratio although the air-fuel ratio is the rich air-fuel ratio. However, when the sulfur poisoning recovery process is carried out for the NSR catalyst while setting the air-fuel ratio of the exhaust gas, for example, to about 14.3, a part of the sulfur component occluded by the NSR catalyst cannot be removed, and the part of the sulfur component remains. When the sulfur component remaining in the NSR catalyst is gradually increased, the purification performance of the NSR catalyst is gradually lowered thereby.